1. Field of the Invention
The present invention relates to an optical amplifier, a method for optical amplification and an optical transmission system to be used in optical communication or optical information processing. In particular, the invention relates to an optical amplifier, and a method for optical amplification for generating forward and backward pumping light components from a pumping source and an optical transmission system using the optical amplifier.
2. Description of Related Art
In a long distance optical communication system, a method to place an optical amplifier on an optical fiber transmission line is generally introduced as one of the means to provide communication for longer transmission distance. In an optical distribution system for distributing optical signal to a number of points, the optical amplifier is now widely used to compensate distribution loss.
Conventionally, as an optical amplifier for directly amplifying the optical signal, an optical fiber amplifier and a semiconductor optical amplifier has been known. The optical fiber amplifier uses rare-earth doped optical fiber in the core as an amplification medium. The semiconductor optical amplifier is based on stimulated emission phenomenon with in semiconductor. At present, the optical fiber amplifier is generally used as the optical amplifier for commercial purpose.
There have been proposed various types of arrangement as optical fiber amplifier. In one of the examples of the arrangement, pumping light is propagated in two directions, i.e. forward and backward directions, in an erbium-doped optical fiber (xe2x80x9cEDFxe2x80x9d), which serves as an optical amplification medium. In this arrangement, two sets of laser diodes are used for forward pumping and backward pumping respectively.
In general, when intensity of forward pumping light is increased, noise characteristics of the output signal of EDF are improved. Also, when intensity of backward pumping light is increased, output power of EDF can be increased. For this reason, in this optical amplifier, it is possible to achieve optimal noise characteristics and optimal output power to match the desired operating conditions by separately controlling forward pumping light intensity and backward pumping light intensity.
In the optical amplifier, the pumping light source is one of the most expensive components. For this reason, it is desirable to design the system in compact size and also to design the pumping sources as a single unit for the purpose of providing the system in compact size and to reduce the cost of the system. That is, it is desirable to use the forward pumping source and the backward pumping source commonly as a single unit.
In case pumping is performed in forward and backward directions using a single pumping light source, the pumping light outputted from the pumping source is divided by an optical coupler such as an optical directional coupler. One of the divided pumping light components is used as forward pumping light, and the other is used as backward pumping light, and these are inputted to EDF.
In case the range of the level of input signal light is extensive or in case an optical amplifier is used in an optical repeater transmission system using wavelength division multiplexing, the required noise characteristics and output power vary according to the number of input channels, wavelength of input channel, input signal intensity and output signal intensity as required. As a result, optimal forward pumping light intensity and optimal backward pumping light intensity are different.
However, in an optical amplifier using a single pumping source by dividing it, intensity ratio of the forward pumping light to the backward pumping light is fixed to a value determined in the designing stage. For this reason, in an optical amplifier using a single pumping light source, it is difficult to control in such manner that optimal forward pumping light intensity and optimal backward pumping light intensity can be obtained at the same time. In this respect, it has been difficult in the past to obtain optimal noise characteristics and optimal output characteristics at the same time to match input/output conditions by a single pumping source.
For instance, in an optical amplifier for wavelength division multiplex transmission, the required output optical power varies extensively. Specifically, in case a signal of maximum number of channels is inputted, high output power corresponding to total sum of light signal intensity of all channels is required for the amplification medium. For this purpose, it is necessary not only to increase output of the pumping source at the time to input maximum number of channels, but also to design the ratio of the backward pumping light (when the pumping light is divided to backward pumping light and forward pumping light) to a higher value to match the maximum number of channels in the designing stage.
On the other hand, in case a signal containing minimum number of channels is inputted, high output power is not required for the amplification medium. For this reason, backward pumping light output intensity of the pumping source is decreased in order to have lower intensity of the backward pumping light. As a result, the intensity of the forward pumping light initially having a low dividing ratio is further decreased. Thus, noise characteristics of the output signal become deteriorated.
It is an object of the present invention to provide an optical amplifier, by which it is possible to obtain optimal noise characteristics and optimal output characteristics to match input/output conditions using a single pumping source.
The optical amplifier according to the present invention comprises a pumping source for outputting a pumping light, a first optical dividing device for dividing said pumping light to a first and a second divided pumping light components, an amplification medium, to which said first and said second pumping light components are inputted, and a dividing ratio regulator for regulating power ratio of said first and said second divided pumping light components.
The method for optical amplification of the present invention comprises an optical dividing process for dividing a single pumping light at a given ratio to generate a first and a second divided pumping light components, and pumping light injection process for injecting said first and said second divided pumping light components to an amplification medium.
The optical transmission system of the present invention comprises an optical transmitter for converting a signal to be transmitted and for transmitting said signal, a transmission line optical fiber for transmitting said optical signal, at least one optical amplifier arranged in said transmission line optical fiber, and an optical receiver for converting output light from said transmission line optical fiber to an electric signal and for outputting said signal, whereby the optical amplifier comprises the optical amplifier as described above.